BEIRUT (REUTERS) - The United States called on Saturday (Aug 15) for a transparent and credible investigation into the massive port blast in Beirut that killed 172 people and said Lebanon could never go back to the days "in which anything goes" at its ports and borders.

The Aug 4 blast, which the authorities say was caused by more than 2,000 tonnes of ammonium nitrate that had been unsafely stored at the port for years, injured 6,000 people, ruined entire neighbourhoods and left 300,000 homeless.

"Seeing it on television is one thing, seeing it up close is another. It's really overwhelming," Mr David Hale, US Under-Secretary of State for Political Affairs, said after visiting the port.

"We can never go back to an era in which anything goes at the port or the borders of Lebanon that had to contribute to this situation," he said.

He added that FBI agents would be arriving this weekend, at the invitation of Lebanon, to help find out what exactly happened and what led to the explosion.

The blast has fuelled anger at ruling politicians who were already facing heavy criticism over a financial meltdown that has sunk the currency, demolished the value of savings and left depositors unable to withdraw their money.

Some Lebanese doubt the authorities can carry out a proper investigation and say foreign countries should intervene.

"We can't trust this government. They will lie to us. They should form an international committee to investigate this,"said businessman Jimmy Iskandar.

President Michel Aoun has said a probe will look into whether the cause of the blast was negligence, an accident or "external interference".

"They won't do a thing in an investigation and the whole world knows that," said painter Mohammed Khodr as he helped repair a restaurant damaged in the blast.

The heavily armed Iran-backed Lebanese group Hizbollah, which is listed as a terrorist organisation by the US, said on Friday that it would wait for results of the official Lebanese investigation into the blast.

But if it turns out to be an act of sabotage by Israel then it would "pay an equal price", Hizbollah leader Sayyed Hassan Nasrallah said in a televised address.

Israel has denied any role in the explosion.

Mr Nasrallah also said his group was against an international investigation because its first purpose would be to "distance Israel from any responsibility for this explosion, if it had responsibility".

He said the participation of the FBI in an investigation would serve the same purpose.

POLITICAL VACUUM

The explosion has pitched Lebanon into a new political vacuum since the resignation of the government, which had formed in January with backing of Hizbollah and its allies including Mr Aoun.

Lebanon's most senior Christian cleric said the Lebanese people and the international community had run out of patience with ruling politicians.

In his strongest intervention yet since the blast, Maronite Patriarch Bechara Boutros Al-Rai also said the church reserved the right to veto any proposals that further jeopardise Lebanon.

His comments in a sermon were reported by Lebanese broadcaster LBC.

The Maronite church exercises political sway in a country where the head of state must be a Maronite, the prime minister a Sunni Muslim and the parliament speaker a Shi'ite Muslim.

The United Nations launched a $565 million (S$774.9 million) aid appeal on Friday.

Priorities included stabilising the grain supply, UN humanitarian coordinator Najat Rochdi said on Saturday, after the explosion destroyed Lebanon's only port-based grain silo.

Six hospitals and more than 20 health clinics were damaged in the explosion and more than 120 schools destroyed, the UN says.

"We would like to be able to rebuild the three hospitals that were completely destroyed," Rochdi said.

Iran backs Hizbollah and in a visit to Beirut on Friday, Iranian Foreign Minister Mohammad Javad Zarif said international efforts should help Lebanon rather than "impose anything on it".